In a cellular wireless communication network, fixed base stations (referred to as NodeB or eNodeB in LTE) provide wireless communication services across an air interface to a plurality of subscribers' User Equipment (UE) within a geographic area (sometimes called a cell) via modulated radio frequency signals. A variety of factors affect the quality with which UE may receive signals transmitted from a NodeB, as quantified by various metrics, such as Block Error Rate (BLER), Signal to Noise Ratio (SNR), Signal to Interference and Noise Ratio (SINR), and the like. Such factors may include geographic distance of the UE from the NodeB, intervening terrain and structures, meteorological conditions, RF interference from other sources, multipath interference, Rayleigh fading, and other factors. If received signal quality is consistently too poor for reliable communications, a UE is preferably “handed over” to a different NodeB providing better quality signals, with the network diverting the data stream from the poorer (source) NodeB to the better (target) one. If a UE cannot be handed off, and received signal quality continues to deteriorate, the UE goes out-of-sync (also called out-of-service) and the network terminates service to it.
The mechanism for Radio Link Monitoring (RLM), the thresholds defining out-of-sync, and the like, are specified in the 3rd Generation Partnership Program (3GPP) technical specification 36.133, Evolved Universal Terrestrial Radio Access (E-UTRA); Requirements for support of radio resource management, section 7.6, the disclosure of which is incorporated herein by reference in its entirety. In particular, the lower threshold on received signal quality, Qout, which will cause a UE to go out-of-sync, is a received BLER on the Physical Dedicated Control Channel (PDCCH) of greater than 10%. This is valid also when no PDCCH is received; in that case a mapping between the BLER and SNR or some other metric of signal quality must be utilized to set the threshold. As long as the estimated performance of the PDCCH, for a given payload size and allocation size (code rate), expressed as BLER, is less than 10%, the radio link is seen as useful. When the estimated BLER has increased to be larger than 10%, Out-of-sync is declared and the UE ceases to transmit. In particular, the physical layer L1 signals Out-of-sync to higher layers, terminating transmission, when the UE is unable to successfully decode PDCCH at the 10% BLER for the number of 200 ms intervals specified in the parameter n310. This starts a timer t310 (in seconds), the expiry of which (without regaining In-sync) results in L1 reporting link failure.
A higher quality reception is required to return a UE to In-sync (also called In-service). In particular, after Out-of-sync has been declared, an estimated BLER on PDCCH of 2% or less (Qin) is required. The UE must successfully decode PDCCH at this level for the number of 100 ms intervals specified in the parameter n311.
FIG. 1 depicts a graphic representation of received signal quality over time, and depicts the operation of the different Qout and Qin thresholds. Note that FIG. 1 depicts received signal quality as SNR; the UE may measure or estimate the BLER on PDCCH from the SNR of the received signal. Accordingly, the thresholds Qout and Qin depicted in FIG. 1 are SNR levels that correspond to an estimated 10% and 2% BLER on PDCCH, respectively.
Initially, a very high quality signal is received, having SNR1 prior to time A. At time A, the signal quality degrades to SNR2, but is still higher than the Qout threshold. Ideally, this lower signal quality may trigger a handover to another NodeB. If not, when the signal quality degrades below Qout—such as SNR3 beginning at time B—the UE shall declare Out-of-sync within the parameter n310 number of 200 ms intervals for which the reception is below Qout. In the example of FIG. 1, n310=1, and the UE declares Out-of-sync, and starts timer t310, at time C. While the UE remains Out-of-sync, it continuously attempts to receive and decode DL traffic until the timer t310 expires.
At time D, the received signal quality improves to SNR4, which is well above the Qout threshold. However, this is still insufficient for the UE to regain synchronization and service. Rather, the UE must wait until the received signal quality improves to be above the Qin threshold—as depicted by SNR5 at time E. In this example, time E occurs sooner than the expiration of the t310 timer. At this point, the UE must wait for the parameter n311 number of 100 ms intervals in which it continuously decodes PDCCH above the Qin threshold. In the example depicted in FIG. 1, n311=1, and the UE L1 declares In-sync status to higher layers at time F. At this point, the UE is again In-sync, and continues to receive wireless service from the network.
The 3GPP standards for RLM are specified with respect to a reference receiver, which is a UE with two receiver antennas (2 Rx). Various advanced receiver architectures are known, in which the receiver enhancements may be selectively enabled, such as the use of a greater number of receiver antennas, Minimum Mean Square Error (MMSE) and Interference Rejection Combiner (IRC) techniques, advanced cancellation of neighboring cell interference, and the like. For the purpose of discussion herein, a representative advanced receiver is one having four receiver antennas, also denoted as a 4 Rx receiver or simply 4 Rx, although this is not the only type of advanced receiver contemplated. The use of more receiver antennas enables the UE to increase the throughput and enhance coverage in some scenarios, both by improving the link level performance for certain configurations and by allowing the use of up to 4 layers MIMO. The requirements for different advanced receivers have been added to the demodulation performance requirements in 3GPP TS 36.101.
When specifying, e.g., the 4Rx receivers, it will not be required by the UE to always have the 4Rx functionality activated. The idea is to only have it configured when receiving data and in scenarios where there is a substantial gain. The power consumption when four receivers are activated is substantially higher than using two receivers, so the UE is allowed to fall back to two-receiver operation when 4Rx is not needed.
With advanced receiver types, PDCCH BLER should be reduced at typical scenarios compared with the reference receivers. Accordingly, the received SNR, where the PDCCH BLER becomes 10% for Out-of-sync and 2% for In-sync, are reduced by typically 3 dB for 4 Rx, compared to 2 Rx. However, the currently specified RLM is based on 10% and 2% BLER on basic reference receivers. The thresholds in the testing are set as SNR levels, where the estimated PDCCH BLER performance of the reference receiver is 10% and 2%. No advanced receivers are assumed.
The synchronization tests can be changed to adapt the thresholds to correspond to the estimated BLER performance for PDCCH when an advanced receiver, e.g. 4 Rx, is used. However, the advanced receiver feature, e.g. 4 Rx receiver, is only assumed to be used when data is scheduled to the UE and there is a clear benefit for the link level performance to use 4 Rx. In this case if 4 Rx is used when going Out-of-sync and 4 Rx is assumed when going In-sync again, then the received signal quality still may be below the Out-of-sync threshold for the basic 2 Rx reference receiver, yet the UE achieves acceptable performance due to the benefit of the advanced receiver feature.
When a UE goes Out-of-sync, the network will stop scheduling transmissions to it. Also, the network will not schedule data to a UE directly after it goes In-sync. Wth no data scheduled, there is no reason for the UE to keep the high-power-consumption 4 Rx feature activated, and it will typically switch to a 2 Rx receiver configuration, e.g., for saving battery life. When the UE decides to fall back from the 4 Rx receiver to the 2 Rx receiver, the estimated PDCCH BLER suddenly increases (for the same actual signal reception conditions). This raises the risk that the UE directly will go into Out-of-sync again. Thus, the sync status of UE will not be stable.
The Background section of this document is provided to place embodiments of the present invention in technological and operational context, to assist those of skill in the art in understanding their scope and utility. Unless explicitly identified as such, no statement herein is admitted to be prior art merely by its inclusion in the Background section.